This invention relates to a process and apparatus for avoiding vinyl chloride emissions in batch suspension polymerizations of vinyl chloride.
The production of polyvinyl chloride by the discontinuous (batch) polymerization of vinyl chloride in an aqueous suspension in the presence of monomer-soluble catalysts and suspension stabilizers is well known.
When conducting these processes on a commercial scale, it has previously been necessary to enter the reactor after the termination of a polymerization operation so that cleaning steps could be carried out. The troublesome wall cakings were then removed by means of pressurized-water appliances and/or scrapers.
The development of automatically operating hydrodynamic cleaning apparatus which are also maintained in pressure connection with the polymerization reactor during the holding times thereof between charges and which can, if necessary, be introduced into the reactor, eliminated the necessity of a manual cleaning step and thus the entrance into the reactor. See Austrian Pat. No. 305,724 whose disclosure is incorporated by reference.
In spite of these more recent technical aids, it has heretofore still been necessary in the suspension polymerization method to open the polymerization reactor after the termination of each polymerization step in order to introduce the polymerization catalyst.
Since oil-soluble catalysts which are not water-soluble are utilized in suspension polymerizations, it is impossible to introduce them into the reactor as an aqueous solution. Attempts to add the catalysts via the vinyl chloride conduit resulted in uncontrollable polymerization and clogging of the conduit.
Consequently, it has been necessary to open the reactor in order to introduce the monomer-soluble catalysts and to feed the catalyst system manually.
The operation of opening a large polymerization reactor requires very time-consuming preparations and subsequent measures. Also, in doing so, considerable vinyl chloride emissions are unavoidable in spite of careful precautionary steps.
The operations necessary for effecting the opening of a polymerization reactor are first, after removing the polymer suspension by pumping, the vinyl chloride is withdrawn by the application of a vacuum; then, a pressure equalization is effected by feeding nitrogen or air. The reactor can thereafter be opened and the catalyst introduced. Subsequently, the polymerization reactor must again be evacuated to remove the nitrogen or air. Only then is it possible to introduce the vinyl chloride for the next polymerization batch.
In the case of a large polymerization reactor now being used and having a capacity of 200 m.sup.3, these steps associated with the opening thereof require about 3 hours, which is a significant proportion of the total time necessary for carrying out a polymerization cycle.
However, above all one must consider that the mixture of nitrogen or air and vinyl chloride still present in the reactor after feeding the catalyst must be blown off into the atmosphere, since a separation of the gaseous mixture would be too expensive technically. In this way, for example, up to 100 kg. of monomeric vinyl chloride is emitted into the atmosphere from a reactor having a volume of 200 m.sup.3 prior to each polymerization charge.
These disadvantages of the earlier mode of operation are overcome by the process of this invention.